Silver impregnated activated carbon, (Ag-GAC), is a well known product, used widely for purifying process-water and drinking-water. The function of the silver bound to the activated carbon is to avoid bacterial growth on the carbon bed and in the water in contact with it. Filter cartridges containing Ag-GAC do not kill bacteria during normal filtration rates. For this reason Ag-GAC is described as a Bacteriostatic medium and not a Bactericide.
The maximum admissible concentration of silver in drinking water is: 0.05 mg/L (50 ppb) in the USA and Canada and 0.01 mg/L (10 ppb) in the EEC. Therefore, in order to enable application of Ag-GAC for purifying drinking water, the loading of silver on the carbon is low, usually lower than one percent. The highest silver loading offered commercially is 1.05%.
Recently iodinated and halogenated resins have been introduced as water purifiers capable of "instantaneously" killing bacteria and viruses upon contact between the treated water and the disinfecting resin. The use of equipment containing such disinfecting resins for purifying drinking water has been approved by the US-EPA for emergency and intermittent applications, not for continuous consumption, because disinfecting resins elute minute amounts of iodine and iodides into the treated water, and excessive consumption of iodine/iodide may cause hyperthyroidism. The "adjusted acceptable daily intake" (AADI), of iodine/iodide has been determined to be 1.19 mg/L (Handbook of Water Quality, Standards and Controls by John De Zuane, P. E., Van Nostrand Reinhold, New York 1990). The concentrations of iodine/iodide eluted to the treated water by commercial disinfecting resins vary between 2 to 15 ppm (mg/L), depending on the type of the resin and on the salinity and temperature of the treated water. Therefore, application of equipment containing iodinated resins for purifying drinking water for continuous consumption can be possible only if the drinking water purifier contains means which reduce the concentration of iodine/iodide in the product water to 1.19 ppm and preferably considerably lower.
Iodides can be removed almost absolutely from aqueous solutions by precipitating them with silver ions. The solubility product of AgI at 25.degree. C. is 1.5.times.10.sup.-16, a million times lower than that of AgCl (1.56.times.10.sup.-10) at the same temperature. Therefore, iodides can effectively be eliminated from drinking water via precipitation as AgI, even in the presence of a large excess of chloride ions such as those existing in drinking water. Metallic silver or silver chloride can generate sufficient concentrations of silver ions for precipitating iodides. However, in order to be effective, the surface area of the silver-ion generating solid must be very large. Otherwise it will very quickly be coated with AgI precipitate and become "blind" to the aqueous solution. Granular Activated Carbon (GAC), has a large surface area and can be an efficient "carrier" for silver. Indeed, Ag-GAC has proven to be effective in removing iodide ions from drinking water.
Nevertheless, commercially available Ag-GACs are not practical for removing iodides from water disinfected by iodinated resins because their silver loading is very low. Thus, even if we take GAC impregnated with 1.05% silver (the highest silver loading available commercially), than 300 ml of such Ag-GAC (the amount which can be included in a 10" CounterTop Drinking Water Purifier) weigh .about.150 grams and contain only .about.1.50 gr. (1,500 mg) of silver. This amount of silver can, theoretically, precipitate only 1.5.times.(126.9/107.9)=1.76 gr. of iodides. Practically, the precipitation efficiency is less than 80%. If we assume that the concentration of iodine/iodide in the water disinfected by an iodinated resin is only 3 ppm, then 300 ml. of 1.05% Ag-GAC would be sufficient for treating less than 470 liters of disinfected water, which is too low a capacity. The minimum reasonable capacity for a 10" CounterTop Drinking Water Purifier (which contains a 5.mu. PreFilter, 300 ml iodinated resin and 300 ml of Ag-GAC) is 1,500 liters--the annual drinking water consumption of one person. With 1.05% Ag-GAC we can attain less than one third that value.